The present invention relates to a terminal unit for use in a CATV system in which many such terminal units are connected to a transmitting source center by coaxial cables, and television signals are transmitted from the source center to the terminal units
In general, a CATV system is configured so as to transmit a program produced in a source center or a program existing on the air to terminal units through coaxial cables.
In certain cases, programs produced in a source center or existing on the air are chargeable. When such programs are received by a terminal unit, the terminal unit viewer is charged a predetermined fee.
In order to prevent unauthorized viewing (pirating) of a chargeable program, the television signals of such a chargeable program are subjected to a scrambling operation to render them unviewable. These programs are scrambled in advance at the transmitting source center so that the pictures cannot be received by non-paying television viewers.
In a receiving terminal unit where the viewer is authorized to receive a chargeable program, a descrambled means is provided for recovering the original signal
Various kinds of such scrambling and descrambling systems have been proposed. For example, a so-called GSS (Gated Sync Suppression) system has been used in which a video signal is subjected to a scrambling operation to form a scrambled signal in which horizontal and vertical synchronizing signal portions are compressed by several decibels so that they cannot be detected in an ordinary television receiver. On the other hand, in a terminal unit authorized to view the television signal, the horizontal and vertical synchronizing signal portions of the compressed scrambled signal are restored to their initial levels, and, therefore, the program signal is detectable by the TV receiver.
FIGS. 6A-6E show the stages of a scrambling and descrambling operation according to the above-mentioned GSS system. More specifically, FIG. 6A shows a base-band video signal. The reference symbol H represents a horizontal synchronizing signal. FIG. 6B shows the video signal modulated into an intermediate frequency (IF) video signal Fv which is transmitted after its horizontal synchronizing signal portion has been compressed by a specific rate, as shown in FIG. 6C(a). Compression rates of 6 dB and 10 dB are typical.
This compressed horizontal synchronizing signal portion includes a horizontal synchronizing signal H and a color burst signal C, and has a time width t.sub.1 of, for example, 12 .mu.sec. Further, in the transmitting source center, a key signal K, indicating the compressing operation timing, is superimposed on an FM audio signal Fa (FIG. 6C(b)). The FM audio signal Fa, on which the key signal K is superimposed, and the above-mentioned intermediate frequency video signal Fv are modulated into an RF signal and transmitted to the terminal units.
In a terminal unit, the audio signal Fa is extracted from the received RF signal, and the above-mentioned key signal K is detected from the audio signal Fa. On the basis of the timing of this detected key signal K, horizontal and vertical synchronizing signal portions of the compressed video signal are expanded by 6 dB, 10 dB or the like, corresponding to the compression at the transmitting side, so as to recover the original signal.
The time width t.sub.2 during which the video signal is to be expanded in the terminal unit is made to be, for example, 10 .mu.sec, which is a little narrower than the time width t.sub.1 used at the time of compressing the video signal, thereby ensuring stability of the descrambling circuit and preventing the loss of a picture portion. As a result, the intermediate frequency video signal Fv recovered in the terminal unit is as shown in FIG. 6D.
The intermediate frequency video signal Fv recovered in the terminal unit is then subjected to AM-detection and reduced to its base-band signal component as shown in FIG. 6E. Next, the audio signal Fa is demodulated and subjected to processing, such as volume control, and thereafter is superimposed onto the video signal after frequency-modulating a 4.5 MHz carrier. The resultant signal is again modulated and sent to a television receiver in a home.
As has been mentioned above, in the compressing and expanding processing, during the scrambling and descrambling operations according to the GSS system, the time width t.sub.2 at the time of expanding is made to be a little shorter than the time width t.sub.1 at the time of compressing. This time difference is necessary because, in addition to the above-mentioned reasons, if the expanding operation is performed with the same time width as that of the compressing operation, then, for example, a portion which has not been compressed would be expanded in the terminal unit because of, for example, a slight shift in the timing in the signal processing between the video signal Fv and the audio signal Fa including the key signal K, so that a black level pulse, whose level is equal to that of the horizontal synchronizing signal, would be produced slightly ahead of a horizontal synchronizing signal or slightly behind a color burst signal on a descrambled signal. Therefore, the synchronization process would be negatively affected by this black level pulse to thereby produce a shift in synchronization, causing a problem in that a normal picture would not be reproduced.
On the other hand, an "unrecovered portion", that is, a white level pulse P (as shown in FIG. 6D), which is caused by the difference between the time width t.sub.1 at the time of compressing and the time width t.sub.2 at the time of expanding, will induce luminance changes, turnover noises and other undesirable effects in a television receiver whose performance has deteriorated due to time aging or the like, or in a television receiver which is adjusted somewhat imperfectly.
These undesirable effects are caused for the following reason. In a television receiver, generally, in order to establish a reference level of a luminance signal, a window is set in the back porch portion including the color burst signal C so that a luminance reference level is obtained from the level output obtained by this window portion (pedestal clamping). Therefore, the unrecovered white level pulse P moves into or out of the window under the influence of jitter or the like, causing the above-mentioned problems to occur.
Recently, for the purpose of improving the quality of picture reproduction, some TV receivers employ a black level correction function. This television receiver is configured such that the black level of a color burst portion is detected to decide whether the black level correction should be performed or not. Therefore, there occurs a phenomenon whereby the black level correction is caused to turn on or off due to the fact that the portion of an unrecovered white level pulse P moves into or out of the detection region. Accordingly, with respect to such a television receiver, there is a problem in that the unrecovered white level pulse P may interfere with the operation of this black level correction function.